Fractal character and direct and indirect transitions in photoemission from silver films

We have examined the linear and nonlinear photoelectric response as well as low-temperature scanning tunnelling microscope (STM) images of rough (coldly deposited) and smooth (annealed) Ag films. Our one-photon and two-photon optical excitation wavelengths were chosen to induce photoemission with common total photon energy, polarization and light penetration depth. Direct optical transitions dominate one-photon and two-photon photoemission from smooth films and one-photon photoemission from rough films; this leads to energy distribution curves that are adequately modelled using well established theory. In contrast, photoelectron emission from rough films induced by two-photon absorption is mediated by the excitation of localized surface-plasmon (LSP) modes centred on roughness features. This photoelectric response is strongly enhanced and it involves indirect, momentum nonconserving transitions. STM images of rough films show an apparently fractal arrangement of structures which support the plasmon activity.